In a cellular communication system each of the subscriber units (typically mobile stations or small portable terminals) communicates with typically a fixed base station. Communication from the subscriber unit to the base station is known as uplink and communication from the base station to the subscriber unit is known as downlink. The total coverage area of the system is divided into a number of separate cells, each predominantly covered by a single base station. The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells. FIG. 1 illustrates a cellular communication system 100. In the system, a base station 101 communicates with a number of subscriber units 103 over radio channels 105. In the cellular system, the base station 101 covers users within a certain geographical area 107, whereas other geographical areas 109, 111 are covered by other base stations 113, 115.
As a subscriber unit moves from the coverage area of one cell to the coverage area of another cell, the communication link will change from being between the subscriber unit and the base station of the first cell, to being between the subscriber unit and the base station of the second cell. This is known as a handover. Specifically, some cells may lie completely within the coverage of other larger cells.
All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. A call from a subscriber unit is routed through the fixed network to the destination specific for this call. If the call is between two subscriber units of the same communication system the call will be routed through the fixed network to the base station of the cell in which the other subscriber unit currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a subscriber unit and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.
In a communication system such as the Global System for Mobile communication (GSM), the subscriber units and base stations perform a number of measurements in order to evaluate the performance of the communication link between the subscriber units and base stations. These measurements are used for determination of best serving base station, hand over decisions, power control etc. Furthermore, it is also proposed to perform a number of measurements, which can be used to derive information of the received signals which can be used for other purposes. Notably, it has been proposed to perform location determination of a subscriber unit from measurements performed on the signals transmitted from base stations.
A known method of location determination consists in determining a distance between an object and a number of fixed positions. For example, in a cellular radio communication system such as the GSM communication system, the location of a subscriber unit can be determined by estimating the range from the subscriber unit to a number of fixed base stations with known locations. If the location must be determined in three dimensions, the range between the subscriber unit and at least three base stations must be determined. This is known as triangulation.
It is known to determine the range between the base stations from the time of arrival of signals transmitted between the subscriber unit and the base station. For example, if a signal is transmitted by a base station to the subscriber unit, the time of arrival will depend on the propagation delay which is proportional to the distance travelled by the radio signal. If the time of transmission is known, the relative time of arrival can be calculated and this will correspond to the range between the base station and the subscriber unit. Typically, the subscriber units are not synchronised with the base stations, and the location is determined from the difference in the time of arrival of signals from different base stations. In this case, signals from at least one additional base station must be received by the subscriber unit. The same general principle applies when location determination is based on transmitting signals from the subscriber unit to the base stations.
In order to determine the location of a subscriber unit according to this method the subscriber unit needs to determine a number of characteristics of the received signals. Notably, the subscriber unit must be able to determine a time of arrival of a signal and also the identity of the base station, which transmitted the signal.
It is known to determine various signal characteristics from comparison of a received signal with a known value of what the transmitted signal is. For example, a GSM subscriber unit can estimate the time of arrival of signals transmitted from various base stations by cross-correlating the received signal by the local replica of the signal to be detected. The training sequence of the GSM bursts can be used for that purpose, whether it is a normal burst, dummy burst or synchronisation burst (SCH) as this information is known for all bursts.
Another example is for a GSM subscriber unit to estimate the identity of the transmitting base station. This is essential for location determination as the subscriber unit has to be sure that the signal it is trying to detect emanates from a given base station and not from one of the co-channel interferers. One possibility is to try and decode every SCH, using the transmitted Base Station Identity Code (BSIC) and comparing this to the known BSIC of the wanted base station. Unfortunately, because the broadcast carrier (BCCH carrier) on which the BSIC is transmitted has a high frequency reuse factor, the strength of the signal from neighbour base stations can be fairly low leading to a wrong decoding of the frame.
The signal to noise ratio will often be sufficient to enable determination of a time of arrival but not sufficient to decode the BSIC and this method is thus very inefficient.
Known methods of estimating signal characteristics such as time of arrival and transmitter identity are thus inefficient, inaccurate and unreliable and an improved method is desirable.